The transport of a passive scalar in magnetohydrodynamic turbulence subjected to mean shear and frame rotation

Abstract

We consider the transport of a passive scalar under the influence of homogeneous turbulence in a conducting fluid that is exposed to a uniform external magnetic field while being sheared in fixed and rotating frames. The mean scalar gradient is assumed to be uniform and aligned with the transverse direction within the plane of the mean shear, while both the frame-rotation axis and the applied magnetic field are aligned in the direction normal to the plane of the mean shear. For any given frame rotation rate, we find that a key parameter determining the structural morphology of the flow is the ratio of the time scale of the mean shear to the Joule time, τshear/ τm. The most drastic modifications to passive scalar transport are observed when τshear≥τm, and these are shown to be linked to changes in the structure of the velocity and scalar fields. In most cases, the observed evolution of the scalar flux coefficients can be explained in terms of structural information provided by the one-point structure tensors, except for high magnetic Reynolds numbers flows, where the alignment of the velocity fluctuation field with the spatially varying local mean magnetic field can, under certain conditions, lead to loss of coherence and suppression of the correlation between the velocity and scalar fields. © 2007 American Institute of Physics.